Controls over the efficiency of methane production in tropical and northern wetland soils

Background/Question/Methods

Methane (CH₄) is a potent greenhouse gas, and wetlands are the largest single global source. Consequently, understanding global controls over the efficiency of CH₄ production in wetlands is essential. Accordingly, we ask the following questions. Are high rates of CH₄ production and low CO₂:CH₄ production ratios under anaerobic conditions in tropical wetlands compared to northern wetlands due solely to warmer temperatures in the tropics, or do tropical wetlands possess unique biogeochemical and microbial properties to explain these dynamics? We additionally ask why many northern wetlands, and particularly deep peat within these wetlands, often grossly exceed the theoretical 1:1 ratio of CO₂:CH₄ production when alternative terminal electron acceptors (TEAs) have been exhausted. These questions are addressed using laboratory production potentials of CH₄ and CO₂, rates of organic and inorganic terminal electron acceptor (TEA) usage, and high through-put genomic and transcriptomic analysis and quantitative PCR of methanogens in a large number of temperate and tropical wetlands. We additionally use FT-ICR-MS and Kendrick mass transform analysis in peatlands from Minnesota, USA and Sweden to identify the organic compounds and likely reaction pathways in deep peat to examine the role of diagenetic hydrogenation in contributing to high CO₂:CH₄ratios.

Results/Conclusions  

Peat and mineral-soil wetlands from Gabon, Africa had uniformly low CO₂:CH₄ production ratios compared to 20 U.S. wetlands ranging from Minnesota to Florida (up to five orders of magnitude difference) when incubated at common temperatures. This result suggests warm temperatures are insufficient to fully explain the highly methanogenic state of many tropical wetlands. pH was an important control over the production ratio in all the wetlands, but low production ratios occurred even at low pH in tropical wetlands. Gene copy number of methanogens and abundance of individual methanogen taxa were good predictors of CH₄ production in the Gabonese wetlands, suggesting an important microbial community component in explaining CH₄ dynamics. Detailed biogeochemical studies of TEAs in northern peatlands indicate that humic substances serve as the dominant TEA in surface peats and may account for the high CO₂:CH₄ ratios observed there. The very low methanogenic potential of permanently anaerobic deep peat requires an alternative explanation, and FT-ICR-MS results suggest that diagenetic hydrogenation of deep peat may explain its very low CH₄ production. Overall, we add important insights into the fundamental controls over the efficiency of CH₄ production in global wetlands.